Buchnera sp., an intracellular symbiont of the pea aphid (Acyrthosiphon pisum Harris), is a close phylogenetical relative of Escherichia coli, and synthesizes a large amount of symbionin, a GroEL homolog. The other heat shock protein homologs, which are not expressed as much as symbionin, have not been studied yet. In this study, we cloned the dnaK and dnaJ genes of Buchnera, and revealed that its DnaK and DnaJ are structurally very similar to those of E. coli. Amino acid residues and motifs proposed so far to be essential for the function of the E. coli DnaK and DnaJ were completely conserved in the Buchnera counterparts. However, Buchnera dnaKJ operon could not fully complement mutations of either dnaK or dnaJ of E. coli. This is probably because of a difference in net charge of DnaK and DnaJ between Buchnera and E. coli, and a unique structure of Buchnera DnaJ that prevents heterologous components from operating in concert. Buchnera dnaK and dnaJ formed an operon whose transcription is governed by a promoter structurally homologous to heat shock promoters of E. coli, although the cellular amount of dnaKJ mRNA was not affected by heat shock. Two inverted repeats flanking both sides of E. coli dnaJ were also found in the gene of Buchnera at the corresponding positions, suggesting that expression ratio of DnaK to DnaJ is regulated in a similar manner in the two organisms.